The present invention relates to a process for preparing 2,2,4,4,6-pentamethyl-2,3,4,5-tetrahydropyrimidine, which is commonly known by the trivial name "acetonin" and which has the formula: ##SPC1##
Acetonin is used to prepare triacetonamine from which, in turn, may be produced a wide variety of triacetonamine derivatives, which are useful as stabilizers for synthetic high molecular weight polymers. As world-wide use of synthetic polymers increases, the demand for triacetonamine derivatives as polymer stabilizers increases correspondingly and there is, accordingly, a demand for a process for preparing acetonin economically on a commercial scale.
Bradbury et al [J.C.S. (1947), 1394 ] disclose a process for preparing acetonin by reacting acetone with ammonia in the presence of a catalyst; various ammonium salts and halides of aliphatic amines were used as catalyst, although it would appear that best results were achieved using ammonium chloride. It was also found that the yield of product could be increased by using calcium chloride as a promoter. Matter[Helv. Chim. Acta., 30, 1114, (1947)] discloses the production of acetonin by reacting acetone and ammonia in the presence of an ammonium chloride catalyst and calcium chloride as a promoter in an amount of 27 mole %. Matter also discloses that calcium bromide, calcium iodide, lithium chloride or lithium bromide may be used in place of calcium chloride in this reaction.
In these known processes, where the catalyst is an ammonium halide or the like alone, the reaction proceeds so slowly that 10 or more hours are required in order to obtain the maximum yield. Moreover, if the reaction is carried out in the presence of a calcium salt as promoter, a viscous mass containing the calcium salt forms during the course of the reaction and this mass sticks to the wall of the reaction vessel, which reduces thermal efficiency and necessitates the use of a powerful stirring device. This, therefore, complicates the reaction procedure. Furthermore, since this mass is insoluble in the reaction mixture, treatment of the product to obtain acetonin free from the calcium salt is difficult and complicated. Accordingly, an improved, commercially attractive, process for the preparation of acetonin is desired by the art.